Water formed as a result of reaction between the humidified ambient air and hydrogen ions in Polymer Electrolyte Membrane Fuel cell (PEMFC) is reused to humidify the incoming air to the required saturation level. Due to this, water in liquid and vapor form enters on the cathode side of the cell. Based on the system temperature, water exists in either gaseous state or liquid state. During the cold start or initial few seconds, most likely the water flows in the liquid state. This leads to maldistribution of water in cells blocking the flow of gases leading to instability in operation due to the cathode starvation. This study is aimed at finding the distribution of liquid water in the stack. Forces due to gravity and surface tension are taken into account. The effect of gravity is more profound due to the orientation of stack.
In this simulation, only cathode-side fluid path is analyzed. The cathode flow path for a single cell is shown in Fig. 1. The single cell is stacked-up to create a stack of 20 cells. Membrane Electrolyte Assembly (MEA) is not considered in simulation. However, the species consumption, production and crossover fluxes are applied at the cathode Gas Diffusion Layer (GDL) to account for cathode reaction and species transfer across the membrane. The species consumption/production is assumed to be varying only along the reactant flow direction. A high current density scenario (>1 A/cm2) is simulated with liquid-water and gas flow at the inlet as 0.05 mL/s and 45 mL/s, respectively.
Numerical Modelling and Results
A 3-dimentional multi-component Eulerian Multiphase model is employed for simulating the liquid water and gaseous reactant. The reactant gas constitutes oxygen, nitrogen and water vapor. The simulation is performed under isothermal conditions of 80oC. Star CCM+, a 3D simulation tool, allows user to define species consumption, production and crossover through external functions. The computation for gases and liquid phases are then computed by 3-D equations. For phase interaction, surface tension force is enabled with coefficient of 0.062 N/m. The contact angle of liquid water with the cell wall and porous GDL is 72o and 110o, respectively . Gravity force is enabled as per the orientation of stack as shown in Fig 2.
The results show that, when the stack is at zero-inclination, inside the header more liquid-water flows towards the cells away from the manifolds. Apart from this liquid-water flow maldistribution in the header, more liquid-water accumulates in the GDL of the first cell (the cell nearest to the manifolds). This is due to the fact that first cell has more flow velocity pushing the liquid-water outside or into the GDL of the cell. When the stack is inclined to 5 degrees, again more liquid-water flows in the header away from the manifolds but more liquid-water is observed in the GDL of the cell away from the manifolds. Also, a highly non-uniform liquid-water flow distribution exists in the channels of this cell. This shows that gravity plays a major role in the maldistribution of liquid-water and surface tension plays a major role in accumulation and flushing of liquid-water.
- E. Gauthier, Q. Duan, T. Hellstern, J. Benziger, FUEL CELLS 12, 2012, vol. 5, 835–847
- Yulii D. Shikhmurzaev and James E. Sprittles, J. Fluid Mech, 2013, vol. 715, 273-282.
- Yuehua Yuan and T. Randall Lee, Springer Series in Surface Sciences 51, DOI 10.1007/978-3-642-34243-1_1
- S.G. Kandlikar, Z. Lu, W.E. Domigan, A.D. White, M.W. Benedict, IJHMT, 2009, vol. 52, 1741–1752
- Yun Wang and Chao-Yang Wang, Journal of The Electrochemical Society, 2006, vol. 153 (6), A1193-A1200
- Yun Wang, Suman Basu, Chao-Yang Wang, Journal of Power Sources, 2008, vol. 179, 603–617